Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

455
FXUS63 KLOT 202331
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 PM CDT Sat Jun 20 2026

.KEY MESSAGES...

- A storm system is expected to bring widespread soaking
  rainfall to the area Sunday through Sunday night.

- An axis of heavy rainfall is expected on Sunday which could
  result in instances of flash flooding and river rises. Favored
  area for heaviest rainfall is south of I-80.

- Dry and seasonably cool conditions are expected to start next
  week before another stormy pattern settles in mid-late week.

&&

.DISCUSSION...
Issued at 200 PM CDT Sat Jun 20 2026

High pressure continues to reside over the Great Lakes this
afternoon which will maintain our tranquil weather conditions
through tonight. Temperatures this afternoon will top out in the
mid to upper 70s despite the breezy northwest winds before
cooling into the mid to upper 50s overnight. While the breezy
winds (gusting around 20-25 mph) will persist through sunset,
winds will ease this evening and become light and variable
overnight.

Heading into Sunday, the weather will turn more active as a
pair of shortwave troughs pivot through the area. The leading
shortwave (currently in central NE) is expected to arrive
towards the mid to late morning hours and will bring with it a
broad area of moderate rainfall. Given that instability during
the morning and early afternoon should be rather meager, little
to no lightning is expected. However, as we get later into the
afternoon and evening the second shortwave will begin to arrive
and be deepening as it does so. The deepening trough will allow
lapse rates to steepen (around 6-7 C/km) and may result in some
embedded thunderstorms developing especially near and south of
I-80.

Associated with the second shortwave will also be a developing
surface low that is forecast to traverse across central IL and
IN Sunday evening. The expected track of the surface low should
keep the northeast IL and northwest IN on the north side of the
system and thus limit the overall thunderstorm coverage despite
the aforementioned steepening lapse rates. That said, the
baroclinic zone associated with the low is expected to be draped
across the area and that will serve as the focus for additional
forcing and an enhancement in rainfall rates. Couple this
forcing with the modest instability and the high moisture
content of this system (PWATs around 1.5-1.75+ inches) and
conditions remain favorable for an axis of heavy rainfall to
materialize. The latest guidance suite continues to favor areas
south of I-80 (particularly into central IL/IN) for the greatest
heavy rain threat, but given the possible convective nature
sudden shifts in this axis may still occur. Regardless, it seems
that a swath of rain amounts in excess of 1 inch to locally 2+
inches will materialize somewhere in our southern CWA. Given
that this area has already seen decent rainfall from last weeks
storms, there is an elevated risk for flash flooding and river
rises as these showers/storms move through. For now we will be
maintaining our Hydrologic Outlook (ESF) for our southern CWA to
highlight this risk because of the lingering uncertainty in
exact rainfall placement, but if confidence grows consideration
may be given to a Flood Watch.

Finally, there is also a non-zero chance for a couple of strong
to possibly severe thunderstorms Sunday afternoon and evening
mainly along and south of a Pontiac, IL to Rensselaer, IN line.
With effective shear only expected to be around 20-30 kts and
the limited surface instability locally, the main severe threat
should stay south of our area into central IL. However, if a
more organized storm does develop then a gusty to locally
damaging wind threat may materialize.

The shortwaves and associated surface low will begin to push
east late Sunday night into Monday morning. This will in turn
allow rain to end from west to east with most locations expected
to be rain-free by mid-morning Monday. Therefore, partly cloudy
skies and seasonably cool conditions can be expected for Monday
with highs peaking in the mid-70s. Breezy northeast winds will
develop behind the departing low and persist through Monday
evening resulting in large waves and dangerous rip currents at
area beaches Sunday night through Monday night. Winds and waves
are expected to subside by Tuesday afternoon as the center of
the surface high passes overhead. So expect tranquil weather to
persist through Tuesday with temperatures being slightly warmer
in the upper 70s. However, onshore winds will keep temperatures
notably cooler (upper 60s) near the lake.

The pattern is forecast to turn stormier towards the middle and
later half of the week as west-southwest flow develops aloft.
This will result in periods of shortwaves traversing across the
Midwest and Great Lakes which in turn means more chances for
showers and thunderstorms. At this range, details on exact storm
coverage, timing, and intensity remains a bit uncertain so if
you have outdoor plans Wednesday onward be sure to check back
for updates. Otherwise, expect temperatures to gradually warm
towards more typical summer values by week`s end.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 631 PM CDT Sat Jun 20 2026

Aviation forecast concerns for the 00Z TAFs:

- Lake breeze likely to bring easterly wind shift at KORD/KMDW
  this evening.

- Rain develops late Sunday morning/midday and persists through
  Sunday evening. Periods of MVFR/IFR  cig/vis expected during
  the afternoon and evening.

- Winds turn light/variable this evening before eventually
  becoming southeast Sunday morning. WInds then become breezy
  from the east and northeast in the afternoon/evening, with
  gusts 20-25 kts likely.

Quiet VFR weather conditions are expected in the near term, with
the only concern this evening being timing of the lake breeze
boundary and it`s easterly wind shift across KORD/KMDW. Modestly
breeze west-northwest flow has held the boundary closer to the
lake through the afternoon, though with the westerly component
now diminishing it`s been making progress westward especially
toward KMDW. Though not a steady progression, would expect a
continued westward shift with KMDW arrival prior to 01Z, and
KORD with a lower confidence prior to 02Z. After a period of
light east-southeast winds post lake breeze, winds across the
area should become light/variable overnight.

Surface low pressure currently over Colorado is then forecast to
track east across the Plains and into the Midwest on Sunday, in
association with a fairly stout mid-level disturbance. Showers
and rain with this system are expected to develop across the
terminals from west to east from mid-morning through midday,
with light-moderate rainfall likely during the afternoon/evening
hours. Model guidance is in good agreement in gradually lowering
ceilings/vis into MVFR during the afternoon, with periods of IFR
(ceilings primarily) possible especially for the Chicago
terminals where onshore flow along with the rain will help
moist/cool the low/levels. Rain is expected through the end of
the TAF period, though may begin to diminish toward the very end
of the current KORD/KMDW 30-hour forecasts.

Wind-wise, southeast winds will increase to around 10 kts in the
morning, eventually backing east and northeast during the
afternoon and evening hours while strengthening and becoming
gusty in the 20-25 kt range.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from Sunday evening through late
     Monday night for ILZ006-ILZ103.

     Beach Hazards Statement from Sunday evening through late
     Monday night for ILZ104.

IN...Beach Hazards Statement from Sunday evening through late
     Monday night for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM Sunday to 1 AM CDT Tuesday for
     the IL nearshore waters.

     Small Craft Advisory from 4 PM Sunday to 4 AM CDT Tuesday for
     the IN nearshore waters.

&&

$$

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